Metallic materials such as pipes, tubes and other structural members of apparatus for chemical plants and petroleum plants arc often used in environments wherein the materials are exposed to alkaline or acidic solution at high temperatures. Superheater tubes, evaporator tubes and structural members of boilers; heat exchanger tubes and condenser tubes of heat exchanging apparatus; and catalyst tubes are used in high temperature, high pressure and corrosive environments.
Boiler tubes, particularly for waste incinerating heat recovery boilers and black liquor recovery boilers (these boilers are referred to collectively as "industrial waste incinerating boilers" hereinafter) are subjected to severe attack of strong corrosive gas such as chlorine gas and hydrogen chloride gas, hydrochloric acid and sulfuric acid at elevated temperatures.
Corrosion resistant alloys are used for the materials of tubes which are exposed to such corrosive environments as mentioned above. For example, Ni-Cr-Fe alloys standardized in JIS G 4903 or 4904 are used for super sometimes. In six alloys standardized in said JIS, alloys named NCF 625 TP or NCF 625 TB which contain 8-10% Mo ("%" in chemical composition means "weight percent" herein) are often used particularly in severe corrosive environments.
NCF 625 TP alloy and NCF 625 TB alloy (referred to as "alloy 625" hereinafter) are Ni-based alloys containing 20-23% Cr, 8-10% Mo, up to 5% Fe and 3.15-4.15% "Nb+Ta" as the major alloying elements, and Al and Ti as the additional elements. The Ni content is restricted to be not less than 58%. The alloy 625 has an excellent corrosion resistance in the extremely severe corrosive environments due to beneficial effects of Cr, Ni and Mo.
Seamless tubes of the alloy 625 for a heat exchanger tube etc. are manufactured usually in a process comprising a step for making a tube blank by a hot extruding process such as Ugine-Sejournet process, and a step of cold rolling or cold drawing of the tube blank. Hot workability of the alloy 625 is so poor that the tube blank made by hot extruding has many surface defects generally. The surface defects should be removed before cold working. Since cold workability of the alloy 625 is not good, cold rolling or cold drawing ought to be performed by repeated passes at a rather small working ratio of each pass.
The productivity of the alloy 625 is low because of the above-mentioned complicated working process, and the low productivity together with high price of raw materials (Ni, Mo, Cr, etc.) makes the alloy 625 very expensive.
Since the alloy 625 originally has precipitation hardening property at about 650.degree. C., toughness of the alloy is considerably reduced during long period use at temperatures over 500.degree. C. Products of the alloy 625 for high temperature use may be broken by thermal-fatigue when they are subjected to heating and cooling cycles. Accordingly the reliability of the alloy 625 products at elevated temperatures is not large, and usage of the alloy is rather limited.
A high Mo nickel-based alloy having a workability better than that of the alloy 625 is disclosed in WO 095/31579 (PCT International Publication). In the alloy, Nb (which has a negative effect on the workability) is restricted to up to 0.5%. In spite of the small amounts of Nb, corrosion resistance of the alloy is said to be as good as the alloy 625. However, the "good corrosion resistance" has been found in a test wherein probes were placed at a specific location in a waste incinerator. Corrosion conditions vary broadly dependent on locations and combustion conditions in the incinerator. The "good corrosion resistance" described in WO 095/31579 is a property which has been recognized under a very specific corrosive condition.
The alloy disclosed in said WO 095/31579 contains Ti as a substantially indispensable component. Extruded tubular products of this alloy have surface defects, because Ti in the alloy combines with N in the air and forms massive TiN on the surface of the product during the tube making process.
Some superheater tubes and heat exchanger tubes are used at elevated temperature after forming, e.g., bending at room temperature. When tubes are installed in a water-wall panel of a boiler as steam generating tubes, they should be welded. The corrosion resistant alloy such as the alloy 625 becomes sensitive to corrosion when it is used at elevated temperatures without any heat treatment after cold forming. The welded portion (more specifically heat affected zone, HAZ) also becomes corrosion sensitive. Therefore, a reliable alloy for practical use ought to have good corrosion resistance after cold working or welding.